Aromatic olefin compounds have been widely used as monomers of polymers or raw materials in the chemical industry. These aromatic olefin compounds have been generally prepared by a dehydrogenation of alkylaromatic hydrocarbons. Usually productivity or selectivity associated with the dehydrogenation is dependent upon the role of hydrocarbons or catalysts. Various diluents or oxidants have been selected and used to prevent deactivation of a catalyst by coke formation and to improve the lifetime of a catalyst in the dehydrogenation. For example, steam is widely used as a diluent to improve activity of a catalyst and the lifetime thereof in the dehydrogenation of aromatic hydrocarbons such as ethylbenzene. Despite a disadvantage in view of thermodynamic equilibrium, hydrogen is usually used as a diluent in the dehydrogenation of hydrocarbons having C3, C4, and C8-C12 to prevent severe coke deposition on the catalyst thereby increasing the lifetime of a catalyst. Further, air or oxygen is used as an oxidant in the dehydrogenation of 1-butene to 1,3-dibutadiene.
Ethylbenzene dehydrogenation, the most widely used dehydrogenation process at present, produces styrene which is a very useful compound in petrochemical industry to be used as a monomer or a starting material for the synthesis of synthetic rubbers, ABS resins, polystyrenes and the like. Its demand is on rapid increase. Styrene has been mainly produced by ethylbenzene dehydrogenation in the presence of an iron oxide catalyst with excess supply of steam to ethylbenzene. Typical catalysts used in the dehydrogenation of ethylbenzene are K—Fe2O3 catalysts. However, there are a few problems associated with the dehydrogenation of ethylbenzene. The ethylbenzene dehydrogenation results in a great deal amount of energy loss in the course of condensing steam used in excess prior to separating the target product from the dehydrogenation. Further, ethylbenzene dehydrogenation is much limited in obtaining a high yield of styrene due to thermodynamic limitation of endothermic reaction.
Therefore, various methods have been attempted to overcome the above-mentioned problems associated with the use of steam during the dehydrogenation of ethylbenzene. The first method involves combining the dehydrogenation of ethylbenzene and the selective oxidation reaction of hydrogen. In this method, the dehydrogenated hydrogen is oxidized by oxygen in order to supply the heat of reaction and to modify the reaction equilibrium, if deemed necessary. Bricker et al. have disclosed a combined process of the dehydrogenation of ethylbenzene and the oxidation reaction of the dehydrogenated hydrogen, performed in the presence of dual catalysts of a dehydrogenation catalyst and a platinum oxidation catalyst in U.S. Pat. No. 4,717,779. U.S. Pat. Nos. 4,418,237 and 4,435,607, assigned to UOP (US), disclose a process for the dehydrogenation of ethylbenzene with a dehydrogenation catalyst in the presence of steam and a selective oxidation of hydrogen in the presence of an oxidation catalyst. In these methods, hydrocarbons are treated with steam and a dehydrogenation catalyst along with a subsequent or concurrent treatment with an oxidation catalyst. It was further suggested that ‘SMART process’, which combines the fundamental concept of UOP and the technology of Lummus, be used with the enhanced process of dehydrogenation of ethylbenzene. However, since there is a danger of explosion with use of oxygen in the oxidative dehydrogenation, these methods have not been yet applied practically.
The second method involves lowering the reaction temperature by means of oxidative dehydrogenation via molecular oxygen, thereby converting the endothermic reaction to one of exothermic reaction. U.S. Pat. Nos. 4,255,283 and 4,246,421 as assigned to the Standard Oil Company disclose an oxydehydrogenation process for ethylbenzene to styrene in the presence of a metal phosphate catalyst composition. There have been reported that zirconium phosphate, cerium phosphate, and carbon molecular sieve as catalysts are used at a temperature of from 300° C. to 500° C. in the oxydehydrogenation of ethylbenzene. However, there is a danger of explosion with use of molecular oxygen, and the selectivity is reduced due to the side reaction of the complete oxidation, partial oxidation, cracking and the like.
The third method, the application of a catalytic inorganic membrane reactor, can improve the conversion of ethylbenzene by favorably shifting the reaction equilibrium and lowering the reaction temperature. In particular, GB Patent No. 2,201,159 suggests the use of a ceramic membrane, which is selectively permeable to hydrogen, can effectively separate hydrogen among the dehydrogenated products. EP Patent No. 438,902 A2 discloses a solid multi-component membrane for use in an electrochemical reactor characterized by a mixed metal oxide material having a perovskite structure. The method is superb in principle but has several disadvantages with use of an inorganic membrane reactor for the expensive construction costs of facilities, and the inefficient heat and material transfer. Thus, it is not suitable for industrial applications.
Therefore, economic and safe dehydrogenation processes of alkylaromatic hydrocarbons are highly demanded which would be able to alleviate the limited equilibrium and to diminish energy consumption with the use of carbon dioxide instead of excess steam.
The present invention introduces a dehydrogenation process of alkylaromatic hydrocabons including ethylbenzene by employing carbon dioxide as an oxidant. Recently, there has been a growing concern of carbon dioxide to be responsible for the global warming caused by the “greenhouse effect”. For the mitigation of global warming due to carbon dioxide, catalytic conversion of CO2 has been extensively studied for the last decade. Most of studies on this field have been concentrated on the utilization of carbon dioxide as a carbon source through catalytic reduction processes with hydrogen as a reductant. However, the catalytic hydrogenation is confronted with some limitations to be commercialized due to the use of expensive hydrogen. On the other hand if carbon dioxide is used efficiently as an oxidant, instead of steam, in the dehydrogenation of hydrocarbons such as ethylbenzene, the dehydrogenation process would be a useful and economical process for saving energy.
However, a small amount of carbon dioxide in ethylbenzene dehydrogenation is known to inhibit the catalytic activity of a dehydrogenation catalyst comprising iron oxide as a major component and K—Fe2O3 as an active oxide component due to the decomposition of active phase in the presence of carbon dioxide (Appl. Catal., 67, 179 (1991)). Thus, use of carbon dioxide was largely limited in the process for preparing styrene by using steam dehydrogenation reaction due to its property of deactivating a catalyst. Carbon dioxide decomposes ferrite compounds, such as K2Fe2O3 or K2Fe22O34 or used as ethylbenzene dehydrogenation catalysts, to K2CO3 and Fe2O3 having much lower activity. It was necessary to utilize catalysts to retain sufficient activity and selectivity when using carbon dioxide in the dehydrogenation process of hydrocarbons. As a result, the inventors of the present invention have already disclosed catalysts supported by iron oxides to increase the catalytic activity with carbon dioxide in the dehydrogenation of hydrocarbons in U.S. Pat. Nos. 6,037,511 and 6,034,032. Sugino et al. reported that the activity of dehydrogenation of ethylbenzene was significantly improved under the flow of carbon dioxide by means of a catalyst having an active carbon carrier impregnated with lithium ferrite oxide (Appl. Catal., 121, 125 (1995)). There are others which also reported that the enhancement effects of the dehydrogenation activity of ethylbenzene were significant with carbon dioxide in the presence of Fe2O3/Al2O3 catalyst (Catal. Lett., 58, 59 (1999)) and an activated carbon-supported vanadium catalyst (Appl. Catal. A., 192, 281 (2000)).
Meanwhile, in recent years, it has been noted that carbon dioxide is not a waste but a useful chemical resource. Therefore, it is a key issue to know how to economically obtain large amount of carbon dioxide to be practically applied for the chemical process. It is under extensive researches to develop how to reduce the volume of carbon dioxide released into the atmosphere since carbon dioxide is thought to be responsible for the global warming caused by the “greenhouse effect”. Conventionally, as a method for separating carbon dioxide gas from an effluent or an exhaust gas, an absorption method is being widely used in the petrochemical process, and a membrane separation method or the like are proposed. However, it is most desirable to directly apply carbon dioxide gas discharged from the exhaust to the reaction process instead of using pure carbon dioxide separated and purified with respect to cost reduction. It is quite normal that a relatively large quantity of carbon dioxide is produced in the field of petrochemical industry. Therefore, it will be highly advantageous in many respects such as transport charges, reduction of expenditure and the like if facilities to perform the dehydrogenation are built in an area adjacent to the release of carbon dioxide.